To complete the final step of the quest, you must fight brawlys medicham. In order to start this fight you have to have pure power OT jolly medicham max happy and ONE POKEMON in party. The only way to get to Mt Pyre is via surf, which requires a Pokémon in your party than can learn surf which medicham cannot.
All of this comes together to form a situation where you are forced to purchase a surf mount in order to progress in game, which doesn't seem as intended.
halp my medicham be a big boi